Coating metal surfaces with refractory metals



A. R. GLOBUS 3,211,572

COATING METAL SURFACES WITH REFRACTORY METALS Oct. 12, 1965 Filed March27, 1963 INVENTOR ALFRED GLOBUS BY W l Qywma ATTORNEYS United StatesPatent O 3,211,572 COATING METAL SURFACES WITH REFRACTORY METALS AlfredR. Globus, Forest Hills, NX., assignor to Consolidated AstronauticsInc., Long Island City, N.Y., a

corporation of Delaware Filed Mar. 27, 1963, Ser. No. 268,440 7 Claims.(Cl. 117-50) This invention relates to new and useful improvements inthe coating of metal surfaces with refractory metals.

The invention more particularly relates to a process for depositingrefractory metals, such as tungsten, tantalum, molybdenum, and the likeon porous metal surfaces, such as sintered iron surfaces and the like.The invention also relates to certain novel products produced by thisprocess.

The invention and its objects will be fully understood from thefollowing description read in conjunction with the drawing which is aflow-sheet depicting the process in accordance with the invention.

In accordance with the invention the metal surface to be coated is firstimpregnated with a molten light metal, such as sodium, potassium,cesium, rubidium, calcium, magnesium or lithium. The impregnated surfaceis then contacted with a refractory metal halide vapor under reactionconditions of elevated temperature and preferably reduced pressurecausing exchange of the halogen atom between the refractory metal andlight metal, depositing the refractory metal and converting the lightmetal to the halide salt.

The starting metal surface to be coated may be any metal surface whichis sutliciently porous to allow impregnation with the molten lightmetal, at least to a superficial depth. Preferably, the porous metalsurface is the surface of a sintered metal article made by theconventional powdered metallurgy techniques, as for example, a sinterediron article made by pressing iron powder to a density short of thetheoretical, and then sintering in the conventional manner in order todevelop adequate shape followed by final finishing, if necessary ordesired.

In addition to sintered iron surfaces, the process in accordance withthe invention is also applicable for the coating of the surfaces ofother sintered articles, as for example, porous stainless steelcrucibles, chromiumcobalt alloy jet turbine blade and pump parts ofchromium iron.

The metal part to be treated in accordance with the invent-ion isimmer-sed in a molten bath of the light metal, such as the sodium,potassium, lithium or calcium so that the light metal impregnates thesurface to at least a superficial depth and preferably to a depth ofabout 1 to 2 mm.

The actual depth of penetration depends upon the porosity of thesurface, the pressure of the molten liquid as is determined by the depthof the bath and whether external, such as inert gas pressure is applied,and the capillary and wetting effect of the light metal on the metalsurface.

The impregnated surface is then contacted with a refractory metal halidevapor under conditions of tem- 3,211,572 Patented Oct. l2, 1965 peratureat which a reaction will take place with an exchange of the halide atomsbetween the refractory metal and the light metal, reducing therefractory metal to metallic form and converting the light metal to thehalide salt. Thus, for example, when employing parts made -of iron whosesurface is impregnated with sodium and contacted with the vapor ofmolybdenum pentachloride, the following reaction takes place:

Suitable refractory metals include any high melting heat, wear orchemical resistant metals, which are available in the form of halidevapors under practical operational conditions. These include: Tungsten,tantalum, molybdenum, zirconium, chromium, columbium, vanadium, and alsoboron and silicon.

Examples of the halides include: fluorides, chlorides, bromides andiodides.

The reaction at ordinary pressures generally requires temperatures inexcess of 800 degrees C., as for example temperatures between 900 and1000 degrees C., but under vacuum, as for example a vacuum between aboutl and 25 micron/Hg, this temperature may be reduced to as low as 500 or600 degrees C. due to the fact that the volatilization of the alkali oralkaline earth metal becomes appreciable at these temperatures undervacuum as does the halide formed. The light metal halide formed mustdiffuse from the surface, as a complete coating of this halide on thesurface will block the reaction from proceeding further. Under thereaction conditions the halide of the reactive metal is removed asrapidly as it forms since it is volatilized and `carried away by thestream of excess refractory metal halides. For this reason while thereaction proceeds rapidly and satisfactorily with sodium or potassium atordinary pressures, it is preferable to use a fairly high vacuum, as forexample 1 to 25 micron/Hg when using calcium or magnesium.

It is preferable to use a halide which will produce a soluble salt inorder to facilitate the cleaning of this salt after the reaction, as forexample, by washing with water. For this reason, when using calcium, thechloride or bromide is preferable to the fluoride as the fluoride saltis not water-soluble. Also the halides having the metal in the lowestvalence state possible to effect higher proport-ional deposition arepreferred.

It is also possible in accordance with the invention to effect thecoating with more than one refractory metal by utilizing a mixture ofrefractory metal halide vapors or by contacting the impregnated :surfacewith several different vapors `in sequence. Thus, for example, by mixingthe vapor of tungsten and molybdenum chloride, the subsequent metallicsurface will be coated with mixed tungsten and molybdenum which,however, may not be a true tungsten-molybdenum alloy. The surface,however, appears uniform and the acid resistance of the surface isextremely marked and will show no signs of etching even after immersionin concentrated hydrochloric acid at room temperature for 24 hours.

It is preferable in accordance with the invention to pass the refractorymetal halide vapor in Contact with the impregnated surface in an inertgas stream, as for example a stream of argon.

i to any appreciable degree,

Most preferable is the use of a halide content of by weight of the gas,but for greater alloying it is possible to use as little as 3% byWeight, or for faster coating to use pure halide. The preferredprocedure is to pass argon or helium over the halide of the refractorymetal held at an elevated temperature so that the argon will entrainseveral percent by weight of the halide. This is allowed to diffuse intoa chamber in which the parts are placed which is maintained at thedesired temperature. As the argon-halide mixture is introduced, gradualdeposition of the refractory metal of the halide takes place with theformation of an increased quantity of alkali halide in the gaseousphase. The more concentrated the refractory metal halide in the gaseousphase, the more quickly the deposition will take place and the lessdiffusion through the metal of the base part.

Complete reaction of all the alkali metal is necessary as otherwise theresistance of the base metal is detrimentally affected. Under highvacuum, using sodium the reaction is completed in 2 hours, but atordinary pressures it may take 16-20 hours. The time is also a functionof the gas diffusion of the halide of the refractory metal to the partssurface and the alkali halide away from the same.

Several methods can be used alternatively to determine .when thereaction is complete, depending on the metal being deposited. One isimmersion in diluted hydrochloric acid (room temperature). If bubblesappear on sample, reaction is not complete. Brinnel hardness tests areapplicable to the harder metals. A ferrie chloride staining technique issatisfactory for coatings to be used as protective surfaces.

The reaction is preferably effected in an enclosed reactor lined with aceramic, such as dense alumina, zirconia,

etc. Greater efficiency may be obtained by using reactors of greaterlengths or those set in series. Excellent results are also obtained whenusing rotary reactors which are rotated slowly.

It is possible to use the reactor for both steps, i.e. the alkali metalimpregnation .and the subsequent refractory metal halide vaporcontacting. In this case the reactor is rst lled With the alkali metal,then emptied of the excess alkali metal and used for the reaction withthe refractory halide.

The waste gas from the reactor can be washed with Water to recover therefractory metal by precipitation as an insoluble acid or various saltsof low solubility which may then be reconverted for re-use. Totalrecoveries of the refractory metals may be of the order of as high as90-96%.

The coating obtained in accordance with the invention because of thegradual deposition of the metal and the comparatively high temperaturesemployed is diffused into the surface at some depth and is often alloyedto a great degree. The coated surface is essentially the pure depositedmetal with a gradual transition from refractory metal through variousalloyed compositions to pure base metal in the direction away from thesurface toward the core. This is particularly true where the initialsurface treated is relatively porous, as for example, in connection withsintered iron. The coatings obtained are thus entirely distinct fromthose obtained by ordinary plating, as for example, electrolysis orvacuum sputtering which produces a metal coating which is essentially inthe form of a separate coating layer. When the initial metal treated isof a material, as for example, stainless steel, products of highchemical and heat-resistance are produced. Even with superficialpenetration in the starting temperature excellent coatings are obtained.Since, however, in this latter case, diffusion does not take place suchcoatings are often coarsely crystalline and rough and are often wellsuited for the production of solid catalysts where this rough surface isof advantage.

The process in aCCOrdance with the invention may be With it micron/Hg.

used wherever it is desirable to form coatings of refractory metals, asfor example, heat and chemical resistance purposes on articles. Thus,for example, coatings may be produced on titanium, beryllium, etc. whichare of excellent resistance to chemicals and heat.

The following examples are given by way of illustration and notlimitation.

EXAMPLE l A sintered iron plate is immersed in a molten bath of sodiumso that the same is impregnated with sodium to a depth of about 11/2 Theimpregnated plate is then placed in a closed chamber lined with densealumina which is maintained at a temperature of about 950 C. A stream ofargon containing 15% by weight of MoCl5 is passed into the chamber incontact with the iron plate. After 20 hours the plate is removed. Theplate has a coating of molybdenum the concentration of whichprogressively increases toward the surface with the surface constitutinga pure molybdenum coating.

EXAMPLE 2 Example l is repeated, except that the sodoium impregnatediron plate is maintained in a alumina-lined high vacuum chambermaintained at a vacuum of about 10 The temperature is maintained atabout 600 C. and the plate removed after about 3 hours.

EXAMPLE 3 Examples l and 2 may be repeated using in place of the iron;(a) porous stainless steel; (b) chromium cobalt alloy; (c) chromiumiron; in place of the sodium; (a) potassium; (b) cesium; (c) rubidium;(d) calcium; (e) magnesium; and (f) lithium; and in place of themolybdenum chloride; (a) tungsten chloride; (b) tantalum chloride; (c)zirconium chloride; (d) columbium chloride; (e) vanadium chloride; (f)boron chloride; and (g) silicon chloride, or the correspondingtiuorides, bromides and iodides. In each case the base will be coatedwith the refractory metal corresponding to the halide.

While the invention has been described with reference to certainembodiments, various changes and modifications which fall within thespirit of the invention will become apparent to the skilled artisan. Theinvention therefore, is only intended to be limited by the appendedclaims or their equivalents wherein I have endeavored to claim allinherent novelty.

I claim:

1. A process for depositing refractory metal on a sintered porous metalsurface which comprises impregnating the porous metal surface with amolten light metal selected from the group consisting of sodium,potassium, and lithium, and thereafter contacting the impregnatedsurface with an inert gas stream containing a halide vapor of arefractory metal selected from the group consisting of tungsten,tantalum, and molybdenum under reaction conditions of elevatedtemperature sufficient to cause exchange of the halide atoms between therefractory metal and light metal, depositing the reduced refractorymetal as a surface coating diffused into the sintered porous metalsurface forming an alloy therewith and converting the light metal to ahalide salt.

2. Process according to claim 1 in which said contacting of theimpregnated surface with a refractory metal halide vapor is effectedunder vacuum.

3. Process according to claim 1 in which said light metal and the halideof said metal halide vapor form a soluble salt.

4. Process according to claim 1 in which said inert gas is argon.

5. Process according to claim 4 in which said argon has a concentrationof refractory metal halide vapor of about 10-25% by weight.

6. Process according to claim 1 iu which said porous 2,671,954 metalsurface is a sintered iron surface. 2,689,807 7. Process according toclaim 1 in which said porous 2,801,462 metal surface is a surfaceselected from the group con- 2,915,384 sisting of titanium ferrous metalchromium alloy and 5 2,930,712 cobalt alloy surfaces. 3,061,4623,071,491

References Cited bythe Examiner UNITED STATES PATENTS Lewin 29-182.1Kempe 117-107.2 Wagner et a1. 29-182.1 Walsh 117-107.2 X Homer et al117-50 Samuel 117-107.2 Horn et a1 117-50 JOSEPH B. SPENCER, PrimaryExaminer.

1,306,568 6/19 Weintraub 117 107 2 10 RICHARD D- NEVIUS, WILLIAM D-MARTIN,

Examiners.

1. A PROCESS FOR DEPOSITING REFRACTORY METAL ON A SINTERED POROUS METALSURFACE WHICH COMPRISES IMPREGNATING THE POROUS METAL SURFACE WITH AMOLTEN LIGHT METAL SELECTED FROM THE GROUP CONSISTING OF SDIUM,POTASSIUM, AND LITHIUM, AND THEREAFTER CONTACTING THE IMPREGNATEDSURFACE WITH AN INERT GAS STREAM CONTAINING A HALIDE VAPOR OF AREFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN,TATALUM, AND MOLYBDENUM UNDER REACTION CONDITIONS OF ELEVATEDTEMPERATURE SUFFICIENT TO CAUSE EXCHANGE OF THE HALIDE ATOMS BETWEEN THEREFRACTORY METAL AND LIGHT METAL, DEPOSITING THE REDUCED REFRACTORYMETAL AS A SURFACE COATING DIFFUSED INTO THE SINTERED POROUS METALSURFACE FORMING AN ALLOY THEREWITH AND CONVERTING THE LIGHT METAL TO AHALIDE SALT.